1. Field of the Invention
The present invention relates to carrier-type, starch-based corrugating adhesives which contain polyvinyl alcohol. More particularly, the invention relates to a high-speed corrugating adhesive which has a carrier phase comprised of starch, modified starch or dextrin and a cold water soluble, partially hydrolyzed, polyvinyl alcohol. The polyvinyl alcohol is further hydrolyzed in situ by heating in the presence of water, caustic, and the starch, modified starch or dextrin.
2. Description of Related Art
Adhesives used in manufacturing corrugated board are usually comprised of starch, a boron containing compound, caustic, and, optionally, a waterproofing resin (where water resistance is needed) in a water base. The main binder of corrugating paper is the starch which is gelatinized in the corrugating process as it penetrates the paper fiber. The other components, namely, the boron containing compound, caustic, and waterproofing resin are auxiliary agents which modify the basic properties of the starch. Caustic, usually in the form of sodium hydroxide, directly affects the gelatinization point temperature of starch. Gelatinization point temperature is often referred to as gel point or gel temperature and is reduced from its natural value of 165.degree. F. for unmodified corn starch to a temperature in the range from about 138.degree. F. to about 152.degree. F. by the addition of caustic. Caustic also enhances penetration of the gelatinized starch into the surface paper fibers allowing for better bonding.
Boron containing compounds perform multiple tasks. The most important function is developing the adhesive tackiness that is crucial in the formation of the "green bond". The green bond is the bond which holds the components of the corrugated board together until final heat curing of the adhesive system. It relies solely on the viscosity of the adhesive to maintain the integrity of the product during curing. Corrugating machine speeds therefore are limited by the rate of viscosity increase (i.e. rate of green bond formation) in the bond line between the liner and the corrugated medium. The oxygen atoms attached to the boron form stronger bonds, sometimes called boron bridges, between the starch and cellulose hydroxyl groups. Boron containing compounds also act as buffering agents in the presence of caustic and help maintain the viscosity stability of the adhesive paste.
It is known that polyhydroxy compounds other than starch, such as polyvinyl alcohol, will respond with the boron containing compounds in a manner similar to that with starch. Boron containing compounds and polyvinyl alcohol are believed to interact synergistically to form strong bonds. Like other polyhydroxy compounds, polyvinyl alcohol in the presence of starch will develop adhesive tackiness faster in the presence of boron, which means that the corrugator can operate at higher machine speeds.
Water proof or water resistant resins are considered to be an optional component of adhesives, and although some corrugating plants may not need them, most plants are prepared to use them when needed. Many types of thermosetting resins are suitable. The most common of them are derived from urea-formaldehyde, ketone-formaldehyde or melamine-formaldehyde. When heat and pressure is applied to them in a corrugating machine, they will condense to form an excellent water resistant network. When polyvinyl alcohol is added to adhesives containing these resins, it participates in cross-linking which will improve the water resistant bonds.
In the corrugating process, adhesive is commonly applied to the tips of the flutes of a corrugated medium. Then a non-corrugated flat paper liner is applied against the adhesive coated flutes as they pass between a corrugating roll and a pressure roll. The resulting product has the corrugating medium on one side and a flat liner on the other side and is called a sinqle-faced portion. The single-faced portion may be used "as is", or adhesive may be applied to the flute tips of the single-faced portion and a second flat sheet can be applied in the same manner as the first in what is called a "double-facer" or "double-backer" operation. The second liner sheet is treated with heat and reduced pressure (relative to the pressure used to make a single-faced portion) immediately following contact with the adhesive.
Starch-based adhesives called carrier-type adhesives are commonly used in processes for manufacturing corrugated paper board. In carrier-type adhesives, a portion of the starch (or dextrin) forms a carrier, often known as the gelatinized phase, which suspends the balance of the starch which is in an ungelatinized state. Under conditions of heat and pressure, the ungelatinized starch is rapidly hydrated and gelatinized to quickly increase the viscosity and adhesivity of the adhesive composition.
Polyvinyl alcohol has been added to starch based corrugating adhesives for many years to increase the viscosity and the strength of the bond between the corrugating medium and the liner material. In Japanese Patent Publication Number 45-19600 to Imoto et al. (July 4, 1970), for example, a suspension of powdered polyvinyl alcohol and ungelatinized starch in an alkaline aqueous solution of gelatinized starch and borax is described for use in producing highly water-resistant corrugated board. Imoto, et al. discloses the use of totally saponified (i.e. fully hydrolyzed) polyvinyl alcohol and water-soluble derivates thereof.
In U.S. Pat. No. 4,094,718 to Czerwin, a fully hydrolyzed polyvinyl alcohol having a fine particle size and low cold water solubles is used in a modified starch-based corrugating adhesive containing borax, alkali and water. The polyvinyl alcohol is in suspension in an aqueous emulsion of the adhesive and is dissolved during the corrugating process when the paper passes between the heated rolls of a corrugating machine.
U.S. Pat. Nos. 4,600,739 and 4,677,145 to Krankkala disclose corrugating adhesives comprised of an aqueous suspension of unmodified starch, caustic, a boric acid compound and a water soluble, partially hydrolyzed, polyvinyl alcohol. Based on the specifications of both patents, these adhesive compositions allow corrugators to attain high line speeds with improved green bond strength. According to U.S. Pat. No. 4,826,719 to Murdock et al., however, it was found that the amounts of soluble polyvinyl alcohol that could be used under the two prior Krankkala patents was limited by the fact that at higher concentrations than claimed, the soluble polyvinyl alcohol tended to form gels as was recognized in Czerwin and Imoto et al.
The Murdock et al. patent discloses a corrugating adhesive based on unmodified starch and containing an alkaline metal hydroxide, a boric acid compound and a cold water insoluble, fully hydrolyzed, polyvinyl alcohol which is heat solubilized in the adhesive composition before application. According to the patent specification, this adhesive composition also allows corruqators to attain high line speeds.
It has now been discovered in accordance with the present invention that very high line speeds can be attained in conventional corrugating machines with a carrier-type corrugating adhesive wherein the carrier phase is prepared by further hydrolyzing a cold water soluble, partially hydrolyzed, polyvinyl alcohol in situ in the presence of water, caustic, and starch, modified starch or dextrin. As a further advantage, gelation problems associated with the use of fully hydrolyzed polyvinyl alcohol starting materials are avoided. It has also been found that the adhesive composition of the invention can be used to make a superior water resistant corrugated board and it is particularly useful for making multi-walled corrugated board a high speeds.